1. Field of the Invention
Methods and systems consistent with the present invention relate generally to an Ethernet passive optical network (EPON), and more particularly, to efficiently distributing an upstream transmission bandwidth in an EPON architecture.
2. Description of the Related Art
To establish subscriber networks from a central office (CO) to buildings or home, various network architectures have been suggested such as x-Digital Subscriber Line (xDSL), Hybrid Fiber Coax (HFC), fiber-to-the-building (FTTB), fiber-to-the-curb (FTTC), fiber-to-the-home (FTTH), and the like. Among these architectures, the deployment of the FTTx (x=B, C, H) can be divided into an active FTTx with an active optical network (AON) architecture and a passive FTTx with a passive optical network (PON) architecture. The PON offers lower costs using a point-to-multipoint topology with passive elements. It is expected that the PON, which is advantageous over broadband services converging audio, data, and broadcasting in the subscriber networks designated as a bottleneck, can aid the implementation and the activation of digital home networks.
The PON connects one optical line terminal (OLT) with a plurality of optical network units (ONUs) using a 1×N passive optical splitter, and constructs a distribution topology of a tree structure.
FIG. 1 shows a construction of a conventional EPON system. The following is an explanation of operations at components of the conventional EPON system in reference to FIG. 1.
The EPON system is configured with one OLT 100 and a plurality of ONUs 120 through 126 connected through a single mode fiber (SMF) in the form of the tree structure. The OLT 100 at a communication company's central office functions like a digital subscriber line access multiplex (DSLAM) that controls a digital subscriber line (DSL) or cable modem services. The OLT 100 broadcasts downstream traffic signals to service subscribers. The ONUs 120 through 126 control and collect upstream traffic signals based their priority, and forward the signals to a transport network such as the Internet and/or a public switched telephone network (PSTN). A passive optical splitter (POS) 110 splits the power of the received optical signal according to the number of the ONUs, and supplies the optical signal with the split power to the first ONU 120 and a passive optical splitter 112. The passive optical splitter 112 operates the same as the passive optical splitter 110.
The first ONU 120 converts the received optical signal to an Ethernet frame which is an electric signal and provides the Ethernet frame to a first end user 130. Also, the first ONU 120 converts data received from the first end user 130 to an optical signal and forwards the optical signal to the OLT 100. The second ONU 122 through the fourth ONU 126 operate the same as the first ONU 120.
As explained above, the data transmission in the EPON system is carried out downstream from the OLT to the ONU and upstream from the ONU to the OLT. The downstream transmission broadcasts data to all ONUs connected to the OLT, and the upstream transmission unicasts data from the plurality of ONUs using a common bandwidth allocated according to the time division multiple access (TDMA) mechanism. To do this, the OLT allocates the common bandwidth to each ONU for the upstream transmission, which is called dynamic bandwidth allocation (DBA). The DBA performed by the OLT for the upstream transmission is a crucial factor to determine efficiency of the EPON system.
As the conventional subscriber network evolves into the optical subscriber network employing optical elements, such as EPON, increased quality of service (QoS) is demanded for various services. Diverse DBA mechanisms for the EPON system have been proposed in response to the QoS support, but they are incapable of providing sufficient support to satisfy the QoS requirement with respect to the diverse services. In the following, conventional DBA algorithms are described.
The conventional DBA algorithms include: 1) a fixed window algorithm that allocates a fixed transmission window (bandwidth) regardless of a user traffic condition, similar to TDMA; 2) a gated window algorithm in which the OLT allocates a requested bandwidth if the ONU calculates the amount of the traffic of the end users queuing at the ONU and requests the corresponding bandwidth allocation; 3) a limited window algorithm in which the OLT allocates the bandwidth not to exceed the maximum allocation bandwidth available to the ONUs if the ONU calculates the amount of the traffic of the end users and requests the corresponding bandwidth allocation; and 4) a credit window algorithm in which the OLT allocates the bandwidth by adding up the requested bandwidth from the ONU and an extra bandwidth. The credit window algorithm can be divided into a constant credit window algorithm and a linear credit window algorithm. The constant window credit algorithm allocates the bandwidth by adding up the bandwidth requested from the ONU and a preset bandwidth. The linear credit window algorithm allocates the bandwidth by adding up the requested bandwidth of the ONU and a bandwidth that is proportional to the requested bandwidth.
However, the above conventional DBA algorithms cannot provide sufficient support for the QoS. For example, the fixed window algorithm may limit the number of subscribers intending to use the services, and is incapable of actively dealing with the rapidly changing traffic due to the absence of the DBA capability. The gated window algorithm is subject to the unfair bandwidth allocation to the plurality of the ONUs because the ONU allocates the bandwidth as requested. In addition, the gated window algorithm lacks the ability to deal with the real-time services in an active manner. In other words, while the gated window algorithm can maximize the efficiency in view of the network operator, the QoS is not provided to some satisfactory degree in view of the end user. The limited window algorithm cannot actively handle the real-time traffic variation due to the limited bandwidth since the maximum bandwidth available to the ONUs is restricted. The credit window algorithm allocates the extra bandwidth to the ONU in order to actively cope with the real-time traffic variation, but may allocate unnecessary bandwidth amount even if the amount of the requested bandwidth is less.